The present invention relates to a method and to a device including an evaluation circuit which permits capacitive sensing in connection with displacement, angle or position sensors so as to enable the position of movable bodies to be determined in a no-contact way.
In most of the cases analogue voltage divider circuits are used for this purpose, which are either designed, according to the known potentiometer principle, as displacement encoders, rheostats or potentiometers, or employed in the form of inductors, for example as differential coils, differential transformers, inductors with short-circuit winding in the form of a tube.
In the case of an analogue resistance potentiometer, a deposit applied on a substrate by spraying or by vapor deposition serves as a resistance path on which a wiper slides in contact relation, thereby being put in a position to pick up different direct voltage potentials of the resistance path, depending on its particular position, and to transmit them, via a collector wiper connected with it, to a collector path where the sensed potential is available for evaluation. Such potentiometers, certain embodiments of which can be used with a very high degree of precision as displacement sensors or pick-offs, may under certain conditions lead to problems, due to the constant contact relation which latter finally also leads to wear in the case of rapid wiper movements, so that there exists a real demand for a system that allows no-contact sensing of the corresponding measured values.
If a capacitive position sensor or displacement sensor of the kind known, for example, from DE 28 26 398 C2 is used instead of the inductive measuring systems, which also works in a no-contact way, but involves certain measuring inaccuracies and, under certain circumstances, also non-linearities, then absolutely significant adulterations of the derived measured values must be expected under certain circumstances, due to the influence of stray capacitances and leakage resistances--a circumstance that is not tolerable in most of the cases.
The capacitive displacement sensor described by DE 28 26 398 C2 comprises a pair of obliquely divided, mutually insulated capacitor plates to which an alternating voltage is applied and between which an intermediate plate, serving as a pick-off being adjustable by the length of the path to be picked off, is arranged and connected to the input of an evaluation circuit via a connection cable. The movements of the pick-off lead to constantly varying forces acting on the connection cable and its connection points; these forces not only result in accelerated aging of the displacement sensor, but have the additional effect, especially due to the changes in the position and displacement of the cable, to cause capacitance variations and varying stray capacitances as well as varying leakage resistances, which constitute a disturbance variable that cannot be specified and, above all, that cannot be compensated in this way.
The evaluation circuit of this known displacement sensor comprises an operational amplifier whose one input is connected via the connection cable to the displaceable intermediate plate, which latter serves as pick-off, and to whose other input the feed-back measuring signal is applied via a resistor connected to ground. The output of the operational amplifier is connected, via a rectifier, to other amplifier elements one of which is configured as an emitter follower. As in such evaluation circuits the arising stray capacitances are in the order of the measuring capacitance, and the input resistance of the amplifier is in the range of the sensor impedance, for usual frequencies, a precise and strictly linear output voltage cannot be expected.
In the case of other capacitive displacement sensors, for which the evaluation circuit is not described at all or only in the form of a simple downstream amplifier (DE 34 41 217), a tightly packed resistance path, showing a meander-shaped or zigzag configuration, is supported on a substrate surface, and a displaceable pick-off element, being arranged at a certain distance from that resistance path and designed as a planar annulus, is provided so as to capacitively uncouple the respective potential and supply it via a connection line to a measuring circuit consisting of a voltmeter. The conductor path is, however, connected to a direct-current supply voltage so that capacitive sensing is possible only during a displacement performed at a correspondingly high speed, while stationary detection of a position cannot be effected, due to the then missing measured value. Here again, disturbance variables are similarly encountered, due to stray capacitances and leakage resistances, which cannot be eliminated.
Finally, it has been known in connection with a galvanometer to connect the pointer of the galvanometer, being a movable element, to an alternating voltage connection, with the pointer moving in a plane at a certain distance above a resistor element, whereby a voltage drop is caused in the latter by capacitive coupling, which voltage drop can then be evaluated to derive the pointer position (U.S. Pat. No. 3,636,449).
Now, it is an object of the present invention to provide a capacitance-based no-contact position sensor and to design its evaluation circuit in such a way as to achieve an especially low sensitivity to disturbances, combined with high measuring accuracy.